The goal of digital audio broadcast (DAB) is to provide a robust method and system for the simplex transmission and reception of high-quality audio (music and/or speech) and ancillary digital data by radio-frequency (RF) signals. The RF signals are generated in a transmitter system, emitted for free-space atmosphere propagation, and received by one or a plurality of receiver systems, which may be mobile. The transmitter may be space-borne or terrestrially located or a combination thereof (i.e. space-borne with terrestrial re-broadcasting). In general, terrestrial transmission is required for adequate mobile receiver performance in areas with dense natural and/or man-made structures because satellite reception typically requires line-of-sight (LOS) propagation due to large propagation path signal loss.
The RF signal for DAB represents digital (bit) information which is encoded in the signal generated in the transmitter system by a modulation method. Unlike conventional analog FM modulation, the information represented by the digital signal (e.g. digitized speech, music, and/or data) is typically unrelated to the characteristics of the transmitted RF signal. A primary goal of DAB is to eventually supplant the existing commercial analog radio broadcast network (i.e. AM-band and FM-bands). Since the primary function of the existing AM-band and FM-band is to provide audio services in the form of music and/or speech, it is presumed that a significant fraction of the encoded digital data represents one or a plurality of digital audio signals.
In many circumstances, the quality of the digital audio signal recovered in the DAB RF signal receiver is expected to be improved when compared to the audio signal recovered from the conventional analog frequency-modulated (FM) signal. Typically, the digital audio signal has i) a higher signal-to-noise (SNR) ratio, ii) a wider (i.e. larger) audio bandwidth, and iii) improved stereo separation (i.e. spatial fidelity) when compared to conventional FM-band reception. For example, conventional analog FM-band broadcast signals have a recovered audio bandwidth of about 15 kHz. While 15 kHz audio bandwidth is substantially greater the audio bandwidth received for commercial AM-band radio broadcast (i.e. 535 kHz-1705 kHz), which is typically less than 10 kHz, 15 kHz is less than the bandwidth of pre-recorded music on the compact-disc (CD) format, which is about 20 kHz.
The improved audio bandwidth advantage of DAB when compared to conventional analog FM-band reception is generally desirable. However, the most significant advantage of DAB is the greater immunity, determined at the receiver, of the DAB signal when compared to the analog FM-band signal to various forms of distortion and interference. The presence of distortion and interference in the recovery of the analog FM-band signal may cause various undesirable artifacts in the audio signal determined at the receiver, for example, static noise, hiss and hum, and clicks. In certain circumstances, the quality of the recovered signal is limited by the effects of background noise. Background noise may be generated by galactic, atmospheric, thermal, and man-made sources (e.g. ignition-noise), for example. Background noise is typically distinguished from distortion which is i) related to the propagation characteristics of the RF signal, for example, multipath, and ii) interference caused by other RF broadcast sources. Background noise is particularly noticeable in the recovered analog FM-band signal when there are pauses or quiet passages in the audio program. In general, the process of analog FM modulation typically improves the robustness of the recovered signal when compared to other analog modulation methods (e.g. amplitude modulation as used in the AM broadcast band) by causing "processing gain" by increasing the occupied bandwidth of the signal. However, the benefits of the analog FM processing gain are substantially eliminated when the source audio signal is silent or quiet.
Reception of the FM-band signal may be significantly disturbed by the effect of multipath propagation. The deleterious effects of multipath are a result of the relatively high RF carrier frequencies, which are characteristic of the FM-band frequency range, and the use of omnidirectional receiving antennas, especially for radio receivers in vehicles. Multipath propagation results from the presence of specular and/or diffuse reflectors in or about the propagation path between the transmitter and the receiver. As a result, multiple signals with varying delay, phase, amplitude, and frequency are received, these signals corresponding to different propagation paths. In general, the deleterious effects of multipath are: i) attenuation of the received RF signal due to destructive coherent interference between paths, ii) dispersion in the received RF signal due to the frequency selective characteristic of multipath, and iii) intersymbol interference between adjacent signal baud intervals. Multipath is typically mathematically modeled as a deterministic linear sum or stochastic function of the transmitted signal and reflections, with background noise modeled independently in the summation.
In addition to background noise and multipath, interference in the received signal may be caused by the presence of other RF signal sources with similar frequencies, including other FM-band transmitters. In circumstances when there are many transmitters, for example, in large urban areas, inter-station interference due to other transmitters operating at the same or similar RF frequencies may the primary cause of signal degradation, except for weak-signal conditions at substantial distances from the transmitter or unusual reception circumstances (e.g. signal shielding in a tunnel).
Because broadcast transmitters may emit high-power RF signals, their operation is often dictated by rules and regulations enacted by governmental agencies. Although the rules are intended to prevent interference between stations, there may be exceptions granted to the adherence to the rules. Furthermore, the presence of noise and multipath may cause reception conditions which are substantially degraded when compared to the nominal conditions which were presumed to exist according to the rules. In the United States, the rules and regulations governing FM-band broadcast are determined by the Federal Communications Communication (FCC).
The general goals of a DAB system are independent of the specific implementation of the DAB system. The purpose of DAB is to provide a simplex communication system which conveys digital audio and ancillary data in the form of RF signals from one or a plurality of transmitters to one or a plurality of receivers. It is desirable that the determination (i.e. demodulation and decoding) of the transmitted DAB signal at the receiver is less degraded by the effects of noise, multipath, and interference when compared to the reception of conventional FM-band broadcast signals for DAB and FM signals with equivalent coverage. The quality of the recovered digital audio signal represented by the DAB RF signal is, when unimpaired, similar to that of pre-recorded CD material (i.e. near CD-quality). However, the direct digital transmission of data in the CD format is inefficient because of the high data rate that is required for the CD representation, which is greater than one million bits per second for stereo signals. In a DAB system, the audio (i.e. speech and/or music) is subjected to source compression, which reduces the required data throughput substantially. Source compression accomplishes a reduction in the required bit rate for audio by exploiting characteristics of the human hearing process in order to remove audio information which is at or below a threshold of perception and whose absence will not, in general, be noticed by the listener.
In a conventional DAB system, the data representing the compressed audio signal is combined with ancillary data and then error-correction encoded, modulated, and emitted as an RF signal at the DAB transmitter. In the corresponding conventional DAB receiver system, the received RF signal is demodulated, error correction decoded, and the effect of the source-compression is reversed to generate a conventional digital audio signal, for example, in the form of a pulse-code modulation signal. The design and implementation of an audio compression and decompression algorithm, known as an audio "codec", is a complex process, but several methods which are suitable for DAB are known. For high-quality stereo digital audio, exemplary algorithms are ISO MPEG-2 layers II and III, MPEG-2 NBC (non-backward compatible), proposals for MPEG-4 audio, Dolby AC-3 reference: K. Brandenburg and M. Bosi, "Overview of MPEG-audio: current and future standards for low bit-rate audio coding," 99th AES Convention, New York, preprint, 4130 F1, Oct. 6-9, pp. 1-26, 1995!, and AT&T PAC reference: N. S. Jayant and E. Y. Chen, "Audio compression: technology and applications," AT&T Technical Journal, pp. 23-34, March/April 1995!. For these codecs, the bit rates for high-fidelity audio are between about 128 kbit/sec and 256 kbit/sec for a stereo signal.
The improvement in received audio quality over conventional FM-band reception brought about by the implementation of a DAB system alone may be insufficient to justify the costs associated with widespread conversion from conventional analog FM-band modulation to a DAB system. Therefore, an important characteristic of DAB systems is the ancillary data capability, which is, in general, unrelated to the digital audio signal. The ancillary data permits broadcasters to increase the range of services which are provided, beyond audio services. Currently, analog FM-band stations may transmit limited associated data services through analog FM subcarrier signals. The subcarrier signals are known as "SCA" signals because of their authorization by the Subsidiary Communication Authorization granted by the FCC. SCA signals are combined with the conventional FM audio signal matrix at baseband so that, unlike DAB, the SCA signal is a component of the transmitted FM signal. A disadvantage of SCA signals is that their bit rate throughput is relatively low, typically less than 30 kbit/sec. Furthermore, the SCA signal is susceptible to the effects of interference and distortion in the same manner that analog FM-band audio reception is degraded because demodulation of the SCA signal typically requires conventional analog FM demodulation as the first step in recovering the SCA signal. For a specific frequency deviation, the robustness brought about by the use of FM modulation diminishes as the baseband modulating frequency is increased. The baseband frequency of the SCA signals is typically somewhat less than the one-sided occupied bandwidth of the analog FM signal (e.g. SCA signals with baseband frequency offsets of 76 kHz or 92 kHz), so that the FM processing gain for the embedded SCA signal is relatively small.
For DAB, preferable characteristics of the ancillary data associated with the digital audio signal are that the recovered ancillary data signal is i) of reasonable bit rate throughput, for example, at least 64 kbit/sec, and is ii) at least as robust to the effects of interference and distortion as the remainder of the digital data (i.e. the digital audio signal) in the DAB signal. While audio compression algorithms may operate correctly with decoded bit error rates as high as 1.times.10.sup.-5 because transient errors may not be detected by the human hearing process, ancillary data may require a much lower decoded bit error rate, typically less than 1.times.10.sup.-7. The range of services which may be implemented with the DAB ancillary data include, but are not limited to: traffic, weather, road conditions, and emergency information; subscriber services such as paging and specialized newscasts (e.g. stock quotes); low-bit rate video and still pictures; electronic-mail and Internet broadcast; navigation; and duplex communication and electronic commerce with a return data-channel accomplished, for example, by cellular telephony.
A fundamental way to characterize a DAB system is by the method which is used to generate the RF signal at the transmitter and to determine the RF signal at the receiver. There are different RF modulation and demodulation methods which may be used to implement a DAB system. One characteristic in distinguishing between methods is whether or not the proposed DAB system implementation requires a new frequency allocation other than the existing AM and FM broadcast bands. The high-power RF signal emissions for DAB will typically be regulated by government agencies in order to mitigate RF interference. Control of the frequency allocation through specific channel frequency assignments and RF emission limits (i.e. average power or power spectrum density) prevents interference from other RF signal sources into the desired DAB signal and vice versa. Since DAB receivers are expected to be a large-scale consumer electronics application, standardized frequency allocations are also to be considered in order to develop economically viable receiver systems.
A DAB system which requires a new frequency allocation, specifically for DAB, and which is not compatible with the existing AM and/or FM bands, is known as a "new-band" system. The Eureka-147 system reference: G. Plenge, "DAB--a new sound broadcasting system. Status of the development--routes to its introduction," (translated) EBU Review--Technical, No. 246, pp. 87-112, April 1991! which is being proposed by various European agencies, broadcasters, and manufactur ers, is an example of a new-band system. The minimum bandwidth for the Eureka-147 system is about 1.536 MHZ. It is not compatible with the existing FM-band broadcast radio frequency allocation, which is discussed subsequently. There are different proposals for the operating RF frequency range of the Eureka-147 system, including L-band (1.47 Gigahertz), and portions of the VHF band which are not presently occupied or occupied by a small number of existing television stations.
The Eureka-147 system is further distinguished from conventional FM-band broadcast systems by being a "single-frequency" network or SFN in its preferred embodiment. In a SFN, a wideband RF digital signal is transmitted which represents information for a plurality of stations; in other words, a multiplex of stations. According to the Eureka-147 system description, various transmitters are established which re-generate the same signal. A SFN implementation is fundamentally different from the conventional FM-band network, where stations operate independently. The coverage of a SFN network may extend for a distance which is substantially larger than the coverage of an individual statio n in a non-SFN network. A new-band SFN DAB system may be desirable in countries or geographic regions which lack an established radio broadcast infrastructure because it allows for centralization of the broadcast industry. However, such a system may have distinct disadvantages in countries, for example, the United States, where the majority of radio broadcasters are not affiliated with the government and where the stations or networks of stations operate independently and competitively.
Another disadvantage of an SFN is that it requires the allocation of significant amounts of RF bandwidth at frequencies suitable for broadcast. In the United States, for example, there is a lack of available (unused) spectrum in the frequency ranges most desirable for terrestrial DAB, which is the VHF region (30 MHZ to 300 MHZ). Operation at higher frequencies requires additional transmission sites and/or higher transmitter power because of increased propagation path attenuation of the RF signal. A SFN involves the construction of a new broadcast infrastructure of transmitters and transmitter antenna systems. This is another disadvantage to the implementation of Eureka-147 in the United States, for example, where there is a substantial pre-existing infrastructure of transmitter sites. Thus, other methods for accomplishing the goals of DAB may be desirable, which do not require a new frequency allocation and which are able to make use of the existing infrastructure.
In the United States, the analog broadcast FM-band is the frequency region between 88.9 MHZ and 107.9 MHZ, inclusive. The region is subdivided into a plurality of one-hundred and one (101) channels (known as channels 200 to 300, inclusive) with a 200 kHz frequency spacing between adjacent channels. A radio station emits a RF signal, which is modulated with analog frequency-modulation (FM) with certain limitations on the permitted frequency deviation, and which is nominally centered (i.e. without modulation) about the allocation frequency (e.g. 90.9 MHZ which is channel 210). The RF signal emissions are required to comply with an emissions "mask". The mask is a power spectrum density envelope. The maximum emitted power varies as a function of the frequency offset from the nominal allocation center frequency. In other words, the emission mask determines the maximum power which may be emitted at a specific frequency (emission), for each frequency within the channel allocation. The signal emitted by the analog FM-band transmitter typically does not substantially exceed the mask frequency-envelope. The mask characteristics are determined by the FCC in the United States.
Prior art FIG. 1 is a graphic representation of the RF emission mask for analog FM-band radio broadcast, according to the FCC Rules and Regulations, Part 73.317. The limit on RF emissions is shown on a graph whose abscissa values are the frequency of the emission and whose ordinate values are the minimum required decibel attenuation, relative to a known power reference. The FM-band station center frequency is indicated by vertical line 1. The power spectrum density associated with the emission mask, as a function of frequency, is the irregular shape indicated by bold line 3. The emission mask limits given by 3 are divided into various regions. Frequency range 5 spans a two-sided bandwidth of 240 kHz about center-frequency 1. Analog FM-band signal 7 is substantially confined to frequency range 5 and hence a FM-band signal is known to have a "occupied bandwidth" of approximately 240 kHz. The FCC mask limits RF emissions (i.e. discrete components of the transmitted RF signal) in frequency region 5 to a power level no larger than zero (0) decibels, referenced to the transmitter's licensed emitted power. In other words, an emission in region 5 may be as large as the licensed power. The actual emitted RF power is a function of the transmitter power and the characteristics of the antenna and coupling system.
Frequency region 9 begins at a 120 kHz offset from either side of center frequency 1 and continues to an offset of 120 kHz. The total width of region 9 is 240 kHz. Region 9 and region 5 are disjoint and adjacent. In region 9, the emitted RF signal is required to be attenuated by at least twenty-five (25) decibels relative to the licensed power. There are further frequency regions which extend beyond region 9 in the definition of the emission mask according to FCC Rules and Regulations 73.317, but the maximum emission power limit is very small, attenuated by substantially more than 25 decibels. A RF DAB signal whose spectrum is confined within the very low power regions (&lt;&lt;-25 dBc) would not have a wide geographic broadcast coverage. The additional regions are specified so that an operating FM-band station is required to have sufficient bandpass filtering at the transmitter to remove the high-order sidebands generated by nonlinear FM modulation, which might otherwise spill-over into the frequency region occupied by an adjacent analog FM-band station.
Analog FM-band signal 7 typically does not occupy all of the available spectrum provided by emission mask 3 in regions 5 and 9 together. Thus, an additional signal may be generated in this region, which is possibly independent of the analog FM-band signal. For example, the generated signal may be a digital communication signal for DAB. The occupied bandwidth of analog FM-band signal 7 is about 240 kHz, extending 120 kHz from the center frequency. Thus, the DAB signal may be generated to approximately surround the analog FM-band signal in the remaining unoccupied portions (11 and 13) of regions 5 and 9. These regions are known as the "upper sideband" and "lower sideband".
Upper sideband 11 is the frequency region between about 100 kHz and 200 kHz higher than center frequency 1. Lower sideband 13 is the frequency region between about 100 kHz and 200 kHz lower than center frequency 1. The frequencies of the edges of the upper and lower sidebands are approximate because the actual occupied bandwidth of the analog FM-band signal varies as the FM baseband signal changes and varies according to whether SCA transmissions are also present, which may differ from station to station. A DAB signal which is generated in upper sideband 11 and/or lower sideband 13 and which is restricted in power to substantially comply with RF emission mask 3 is known as an "In-band On-channel" (IBOC) DAB signal reference: N. S. Jayant, et. al., ibid.!. "In-band" refers to generation of the DAB signal in the conventional FM-band of frequencies, and "On-channel" refers the co-existence of the conventional analog FM-band signal and the DAB signal within the conventional FM-band FCC emission mask, and not necessarily the 200 kHz allocation increment.
The FCC RF emission mask specifies limits on discrete emissions; in other words, the mask specifies the maximum "power spectrum density" (PSD) envelope as a function of frequency. In general, it is difficult to determine compliance with a discrete emission mask limit over an arbitrarily narrow bandwidth, since in certain circumstances, the amount of apparent power may decrease as the measurement bandwidth is made more narrow. To measure an emission for PSD compliance, it is generally necessary to specify the bandwidth in which the emission is measured (i.e. the "resolution" bandwidth). Unfortunately, this characteristic is ambiguous in the existing analog FM-band emission mask definition. The ambiguity is not problematic for the measurement of conventional analog FM-band emissions because the decibel ratio measured between the power of the discrete emission relative to the total power is approximately independent of the resolution bandwidth. However, the IBOC DAB signal which may be generated in sidebands 11 and 13 is unrelated to the analog FM-band signal and is typically not generated with conventional analog frequency modulation. As a result, the PSD ratio measured between the IBOC DAB signal power and the analog FM-band signal power depends upon the resolution bandwidth. As a result, IBOC DAB systems may generate a digital signal which generally follows RF emission mask 3, but which may not precisely comply with the emissions limit, depending upon how the emissions are measured.
The purpose of the RF emission mask is to ensure that the transmitted analog FM-band signal is confined in both signal bandwidth and signal power so that the presence of nearby FM-band transmitters with similar frequencies will not cause excessive inter-station interference. For IBOC DAB, though, there are other considerations which may constrain the practical IBOC DAB signal bandwidth more severely than the analog FM-band RF emission mask. For example, the second-adjacent channel interference circumstance, discussed subsequently, limits the maximum frequency offset of the outer edges of the IBOC DAB signal in sidebands 11 and 13 relative to center frequency 1. Similarly, the maximum power level of the IBOC DAB signal in sidebands 11 and 13 is limited by the amount of noise that can be tolerated in the reception of the conventional analog FM-band signal. The presence of the DAB signal typically causes an increase in the noise level determined in the FM-band receiver because of the close proximity of sidebands 11 and 13 to analog FM-band signal 7. The amount of noise is primarily determined by the intermediate frequency (IF) bandwidth of the bandpass filter implemented in the analog FM-band receiver.
The primary advantages of IBOC DAB when compared to a new-band DAB system, such as Eureka-147, is that an IBOC DAB system i) does not require a new frequency allocation, ii) makes use of the existing independent broadcast infrastructure, and iii) facilitates a gradual migration from conventional analog FM-band broadcast towards an all digital broadcast radio system. In prior art FIG. 1, upper 11 and lower 13 sidebands do not significantly overlap with the frequency range occupied by analog FM-band signal 7. The sidebands are a desirable frequency range for the IBOC DAB signal. When the IBOC DAB signal and analog FM-band signal overlap significantly, then the presence of the IBOC DAB signal may cause significant amounts of noise in the reception of the analog FM-band signal and vice versa. For example, the presence of the IBOC DAB signal may be noticeable in the recovered analog FM-band signal as an increase in the level of background noise. On the other hand, the amount of power in analog FM-band signal 7 is typically substantially larger than the total IBOC DAB signal RMS power, by between 12 and 30 decibels, for example. Then, even when there is relatively little frequency overlap, the disparity in analog FM and IBOC DAB signal power may cause failure of the IBOC DAB signal receiver because of the large amounts of interference caused by the analog FM-band signal.
In the IBOC DAB method and system described by Hunsinger, et. al., in U.S. Pat. No. 5,465,396, the IBOC DAB signal bandwidth substantially overlaps that of the analog FM-band signal. The IBOC DAB signal is generated so that it is phase-orthogonal (quadrature) with respect to the instantaneous carrier frequency of the analog FM-band signal at the transmitter. In order to be orthogonal to the phase-modulated signal, the DAB signal is generated by amplitude modulation of the FM signal (AM, but unrelated to AM-band broadcast radio), and hence the term "AM-over-FM" modulation in the '396 patent. Phase-orthogonal signals may overlap in frequency and yet not cause interference with one another in the respective signal receivers under certain circumstances. However, phase-orthogonal signals are typically very sensitive to the effects of multipath and frequency-selective distortion. In multipath, the phases of the echo signals are typically unrelated to the phase of the line-of-sight signal, so that the echo signals and the LOS signal are not phase-orthogonal. Similarly, in frequency selective distortion, the signal phase-derivative varies as a function of frequency at the receiver so that phase-orthogonality may not be maintained for certain frequency components of the transmitted signal. As a result, IBOC DAB systems such as that described in the '396 patent, which rely substantially on phase-orthogonality in order to prevent interference between the IBOC DAB signal and the analog FM-band signal, may not perform well in mobile reception environments, where multipath is present. The '396 system is sensitive to even small amounts of distortion in the phase-orthogonality property between the IBOC DAB signal and the analog FM-band signal because of the power disparity between the signals.
In general, it is preferable for the IBOC DAB signal to be situated away from those portions of the analog FM-band signal with the most power, which are located near center frequency 1 of the allocation, which is why it is desirable to generate the IBOC DAB signal in sidebands 11 and/or 13, which surround the analog FM-band signal but which are also approximately frequency-orthogonal. Frequency-orthogonal signals are typically disturbed significantly less than phase-orthogonal signals by the analog FM-band signal because the analog FM-band signal may be substantially eliminated by bandpass filtering. Even when there is some amount of separation in the frequency regions nominally occupied by the analog FM-band signal and the IBOC DAB signal, the nonlinear characteristic of analog FM modulation may cause short-duration (i.e. transient) interference as the high-order Bessel function harmonics associated with the analog FM signal may occupy portions of sideband regions 11 and 13 temporarily, especially when the analog FM-band signal is heavily modulated.
The potential for mutual interference between the analog FM-band signal and the IBOC DAB signal limits how close the inner band-edges of the IBOC DAB signal in sidebands 11 and 13 may be situated to the analog FM-band signal when the signals are unrelated in phase (i.e. not quadrature). Typically, the closest practical inner-band edge, in other words, the edge which is closest to analog FM-band signal 7, for both upper sideband 11 and lower sideband 13 of the DAB signal is between about 80 kHz and about 120 kHz away from analog FM-band center frequency 1. The RF emission mask 3 constraint on the outer band-edges, which are the edges furthest away from center frequency 1, is about 240 kHz for both sidebands. However, the potential for inter-station interference, particularly in urban areas where there are many analog FM-band transmitters, results in a more restrictive constraint (i.e. less bandwidth) on the sideband outer band-edges. The possible second-adjacent allocation interference condition, described subsequently, requires that the outer edge of the IBOC DAB signal in sidebands 11 and 13 be no more than about 180 kHz to 220 kHz away from center frequency 1 so that the total bandwidth occupied by the analog FM-band signal and IBOC DAB signal together is no more than about 400 kHz.
In order to understand the effect of inter-station interference on an IBOC DAB system, it is relevant to first consider the circumstance with conventional analog FM-band broadcasting and then determine the relationship between the analog FM-band and IBOC DAB interference parameters. The reception of the signal for analog FM-band broadcast radio stations is degraded by interference from other FM-band stations, whether or not there is a simultaneously transmitted IBOC DAB signal. The amount of received energy which corresponds to signals from other FM-band transmitters may be considerably greater than the amount of background (i.e. non-broadcast) noise. In this circumstance, adequate signal reception may be interference-limited and not noise-limited. The RF signal emitted by an analog FM-band transmitter is, in general, unrelated to the RF signal emitted by another analog FM-band transmitter because the conventional analog FM-band radio infrastructure is not a synchronized SFN such as Eureka-147. The FCC RF emission mask limits the occupied bandwidth and power which may be transmitted. However, restrictions on the occupied bandwidth of the FM-band signal do not provide adequate protection against interference in the circumstance where there are other (interfering) analog FM-band transmitters with the same nominal channel allocation frequency. Therefore, there are additional regulations, which are also determined by the FCC in the United States, that place restrictions on the geographic location of the RF signal transmission sites and configurations (e.g. antenna height). Thus, the regulatory efforts of the FCC act to coordinate the transmission of the RF signals in the analog FM-band in order to limit the amount of interference amongst the transmitted signals through limitations on signal power and bandwidth and by using geographic isolation. In rural areas, for example, there may be only a small number of radio stations (and hence transmitters). Ample frequency ranges in the analog FM-band may exist which would permit the IBOC DAB signal sideband outer band edge to extend substantially beyond 200 kHz away from center frequency 1. However, in urban areas, there may be significant congestion in the analog FM-band due to the presence of a large number of stations.
In the United States, a substantial source of revenue for broadcasters is derived from advertising. Thus, it is desirable for an FM-band station to have a large potential listener audience. The potential size of the listener audience is determined primarily by the transmitter power, the antenna system, and the population density about the transmitter site and to a lesser extent by terrain characteristics. In most circumstances, it is desirable that the FM-band signal be of a sufficient strength over a wide geographic area so that adequate reception of the FM-band signal is possible with a conventional FM-band receiver (e.g. automobile receiver with a vehicle-mounted whip antenna). Various methods are used in the broadcast industry to specify the FM-band signal strength over varying geographic areas. For example, contour maps are used to indicate approximately isotropic RF field-strength curves. The "coverage" of an analog FM-band station may be defined as the furthest contour (most distant from the transmitter) at which reasonable reception is probable, as determined by the expected average received signal field strength. In general, it is not practical to map precise field-strength contours because of local variations in received RF field strength due to anomalies in the RF propagation characteristics, variations in terrain, buildings, and so on, as well as variations in receiver performance. Field strength contours are typically expressed in probabilistic terms. For example, the "50/10" contour is a hypothetical boundary, situated around the transmitter site, at which at least fifty (50) percent of the locations are expected to have "adequate" received RF signal strength for at least ten (10) percent of the time. The coverage of an analog FM-band transmitter is defined as the geographic area within the "50/50" contour; in other words, the field strength contour at which fifty-percent of the locations will have sufficient field strength for fifty-percent of the time. A goal of IBOC DAB is to have a digital signal coverage characteristic which is not significantly less than the coverage of the corresponding analog FM-band transmitter despite the fact that the IBOC DAB signal is transmitted at an RMS power level which is significantly less than the corresponding analog FM-band signal. This is possible because the IBOC DAB signal receiver typically exhibits good receiver performance at a smaller signal-to-noise ratio (SNR) when compared to an analog FM-band receiver.
For a specific transmitter power and antenna system, the coverage of an analog FM-band station is primarily determined by the characteristic attenuation of RF propagation. As the distance from the transmitter to the receiver increases, the received RF field strength decreases. At some distance from the transmitter, the field strength becomes insufficient for reasonable continuous reception. When the reception is limited by thermal noise in the implementation of the receiver, then a more complex receiver implementation with a smaller "noise-figure" (NF) may extend coverage. However, when the reception is limited by interference, a receiver with a smaller noise-figure may have no effect on the coverage. The realized coverage is further compromised when compared to ideal free-space propagation by the presence of obstacles situated between or about the transmitter and the receiver, which may obscure the line-of-sight (LOS) propagation path and/or cause additional RF signal reflections. Then, the effects of multipath propagation (signal echoes) may cause further attenuation in the received signal due to destructive interference between paths with varying phases, amplitudes, and delays.
In addition to the attenuation characteristics of free-space propagation, with and/or without multipath, the received RF signal is degraded by the presence of other RF sources with similar frequencies, for example, signals from other FM-band transmitters. The effects of inter-station interference from other transmitters may cause significant distortion in the desired received signal at distances which are relatively close to the desired station's transmitter. Thus, the coverage characteristic of an FM-band station may be determined more by the physical proximity between FM-band transmitters than by the effects of background noise at greater distances, in certain circumstances. In general, the most severe potential for inter-station interference for conventional analog FM-band stations is caused in allocation circumstances known as "co-channel", "first-adjacent channel", and "second-adjacent channel".
The co-channel circumstance occurs when analog FM-band stations have the same allocation frequency. Thus, the emitted signals overlap one another substantially and frequency-specific bandpass filtering in the receiver is ineffective in separating the signals. As a result, co-channel station transmitters are required by FCC regulations to be separated by a substantial geographic distance. The minimum required separation depends upon the relative emitted signal powers of the stations. For example, in the United States, FCC class "B" transmitters, having radiated powers of up to 50 kW at a 500 foot antenna height, are to be separated by at least 150 miles. The degree to which signals from transmitters may interfere with one another is expressed by the decibel ratio between the desired signal's field strength magnitude and the undesired signal's field strength magnitude, which is known as the "D/U" ratio. The D/U ratio is essentially a measure of receiver SNR in a interference-limited circumstance over a bandwidth which includes both the desired and undesired signals. In most cases, only the geographically closest transmitters for the particular channel allocation circumstance need to be considered. The linear value of the numerator "D" in the D/U ratio is determined by the desired signal's field strength (milliVolts/meter) at the 50/50 field strength coverage contour. The linear value of the denominator "U" in the D/U ratio is the undesired signals' field strength. The undesired signal's linear amplitude is typically determined at the 50/10 field strength contour of the interfering station transmitter. The FCC Rules and Regulations require that the transmitters' effective radiated power and geographic separation be such that the D/U ratio for co-channel interference is at least +20 decibels (dB), which means that the desired signal field strength linear magnitude is at least ten (10) times as large as the undesired field strength linear magnitude at the 50/50 contour for the desired signal's transmitter.
The IBOC DAB signal represents an encoded digital audio signal, together with ancillary data. As a result, the bit error rate determined at the IBOC DAB receiver after error correction code (ECC) decoding must be relatively small for proper operation of the receiver system, for example, less than 1.times.10.sup.-5. Digital communication systems have the advantage, when compared to analog communication systems, that the addition of information redundancy through ECC encoding facilitates the recovery of the signal at the receiver with a very low error rate, even when the signal-to-noise ratio (SNR) is relatively small. At reasonable bit densities (typically less than or equal to about 3 bits/sec/Hz), the required SNR is significantly less than the 20 dB, typically less than about 15 dB. In the co-channel interference condition, the primary source of interference in a received IBOC DAB signal from the co-channel station is not the analog FM-band signal, but the possible IBOC DAB signal which may be generated by the interfering station. The co-channel analog FM-band signal is approximately frequency-orthogonal to the IBOC DAB signal and so it is substantially mitigated, together with the corresponding on-channel analog FM-band signal associated with the desired IBOC DAB signal, by bandpass and/or notch filtering in the IBOC DAB receiver. However, the IBOC DAB signal which may be generated by the co-channel transmitter substantially overlaps the desired IBOC DAB signal in frequency. The ratio of the desired IBOC DAB signal power to the undesired IBOC DAB signal power is the same as that for the analog FM-band circumstances; in other words, the IBOC DAB signal D/U for co-channel allocations is at least 20 dB because the power ratio between the analog FM-band signals and the IBOC DAB signals follow. Therefore, the existing protection ratios established for the analog FM-band co-channel circumstance are more than sufficient for the IBOC DAB signal. As a result, the coverage of the transmitted IBOC DAB signal is not degraded relative to the coverage of the analog FM-band signal due to co-channel interference. On the basis of IBOC DAB D/U ratios alone (in other words, when ignoring the analog FM-band signals), co-channel IBOC DAB transmitters could be situated closer geographically than is presently allowed by the FCC Rules and Regulations for analog FM-band transmitters without causing undue interference.
The first-adjacent circumstance occurs when analog FM-band transmitters differ in allocation frequency by plus one or minus one channel allocations (i.e. .+-.200 kHz). When considering the interference-limited coverage of a specific analog FM-band transmitter, there may be i) one dominant source of first-adjacent interference at either a positive 200 kHz frequency offset or a negative 200 kHz frequency offset from the desired signals' center frequency, or ii) two comparable interference sources at both positive and negative offsets of 200 kHz. The occupied bandwidth of an analog FM-band signal is about 240 kHz, which exceeds the allocation frequency increment. Thus, in a first-adjacent circumstance, there may be some frequency overlap between the desired analog FM-band signal and the interfering analog FM-band signal, particularly when the audio signals are heavily modulated (i.e. causing wide frequency deviation). However, frequency-specific bandpass filtering with a very narrow IF bandwidth (typically less than 200 kHz) is reasonably effective in separating the desired signal from the undesired signal. The disadvantage of narrow bandpass filtering in the analog FM-band receiver is that it causes some distortion in the recovered analog audio signal due to i) the difficulty in implementing high-frequency bandpass filters with steep transition bandwidths while simultaneously preserving good phase and amplitude linearity and because ii) the attenuation of the higher-order nonlinear FM harmonics by the narrow bandwidth filter deleteriously affects the high-frequency response of the FM signal demodulator. For vehicle FM-band receivers, the disadvantages of a narrow bandwidth filter are typically less important than the advantages of a narrow bandwidth filter in combating both the effects of interference and noise. With a narrow bandpass filter less than about 200 kHz wide, the analog FM-band signals in the first-adjacent circumstance are approximately frequency-orthogonal. The FCC Rules and Regulations allow for a relatively high level of first-adjacent interference, with the presumption that the analog FM-band receiver will be implemented with sufficient filtering. The allowable D/U ratio for the first-adjacent circumstance is only +6 dB. In other words, at the 50/50 coverage contour of the desired signal's transmitter, the field strength linear magnitude from the first adjacent interference source(s) may be as large as one-half (1/2) of the field strength linear magnitude of the desired signal. This low D/U ratio for first-adjacent interferers does not afford ample protection even for the analog FM-band signal in certain circumstances, for example, when multipath effects become significant.
The first-adjacent interference condition is a difficult problem for an IBOC DAB system design to overcome because the IBOC DAB signal, which occupies upper and lower sidebands 11 and 13, may significantly overlap the frequency range occupied by the interfering analog FM-band signal, while also having significantly less power than the interfering signal. Then, although the analog FM-band signals are approximately frequency-orthogonal, one of the sidebands of the desired IBOC DAB signal may be substantially disturbed by the interfering analog FM-band signal so that reliable digital demodulation is not possible. The effect is to significantly limit the coverage of the IBOC DAB signal in the direction towards the first-adjacent interfering transmitter.
The second-adjacent interference circumstance occurs when the channel allocation frequencies differ by two channel allocations, namely, 400 kHz. With respect to the center frequency of the desired signal, the dominant source of second-adjacent interference may be a transmitter at a positive frequency offset of 400 kHz or a negative frequency offset of 400 kHz or by transmitters at both positive and negative 400 kHz frequency offsets. In the second-adjacent circumstance, there is no significant frequency overlap between the analog FM-band signals because the occupied bandwidth of each of the analog FM-band signals is substantially less than 400 kHz. As a result, the permissible D/U ratio is very low, -20 dB. The negative decibel D/U ratio indicates that the field strength linear magnitude of the undesired signal may actually exceed the field strength linear magnitude of the desired signal by a factor of ten (10) at the edge of coverage of the desired signal. This is usually not a significant problem in the reception of analog FM-band signals because bandpass filters with a high "Q-factor" (typically implemented in the FM receiver at an IF of about 10.7 MHZ) substantially attenuate the frequency components which correspond to the possible second-adjacent interference sources. However, due to the potentially large field strength of the secondadjacent source when compared to the desired signal field strength, intermodulation distortion (IMD) due to nonlinearities in the implementation of the RF tuner and other components may cause degradation of the received signal.
The second-adjacent circumstance is a much more important factor in the design of the digital signals for IBOC DAB than for analog FM-band signals because the possibility for interference limits the practical outer band-edges of the IBOC DAB signal upper and lower sidebands to be no more than about 200 kHz away from the analog FM-band center frequency.
There are other interference circumstances which arise where the frequency offsets between interfering transmitters are at least 600 kHz apart (i.e. at least three 200 kHz allocations). The cause of distortion in the desired received signal in these circumstances is due substantially to intermodulation distortion for both analog FM-band reception and IBOC DAB reception. Since the bandwidth of the IBOC DAB signal together with the analog FM-band signal is no more than about 400 kHz, signals from transmitters separated by 600 kHz or more will not affect one another in the receiver when filtered adequately, except for intermodulation distortion in, for example, the RF tuner. The interference conditions beyond second-adjacent do not significantly impact the design of an IBOC DAB signal.
The plethora of possible interference conditions is further complicated by the existence of older analog FM-band broadcast station transmitters which may have been established prior to the adoption by the FCC of certain interference-limiting regulations. In certain circumstances, these stations may be subject to "grand-fathering", which allows for non-compliance with some regulations. As a result, the amounts of interference may exceed the FCC limits, described previously, so that the measured D/U ratios are worse (i.e. are lower-valued) than the nominal conditions. In other words, operating analog FM-band transmitters may be geographically located closer to one-another than would be allowed by the present regulations, which is known as a "short-spacing" circumst ance. For analog FM-band reception, the most serious potential for degradation occurs with short-spaced co-channel and first-adjacent transmitters.
Although analog FM-band signals and IBOC DAB signals are both deleteriously affected by interchannel interference due to the co-channel, first-adjacent and second-adjacent circumstances, the cause of the interference and the magnitude of the problem created by the interference differs considerably. With reference to prior art FIG. 2, analog FM-band channel center-frequency 1 and analog FM-band signal 7 are as in FIG. 1. The IBOC DAB signal occupies both upper sideband region 11 and lower sideband region 13, situated at positive and negative frequency offsets between about 100 kHz and about 200 kHz away from analog FM-band center frequency 1, which is designated f.sub.c. The possible first-adjacent transmitted signal in FIG. 2 has analog FM-band channel center-frequency 15, which is shown with a positive 200 kHz offset in frequency compared to center-frequency 1, i.e. f.sub.c +200 kHz. As described previously, it is also possible for first-adjacent interference to exist at a negative 200 kHz frequency offset from center-frequency 1, or simultaneously at both positive and negative 200 kHz frequency offsets. However, the most severe first-adjacent interference, in other words, where the D/U ratios may be +6 dB, typically occurs on only one side of center-frequency 1 at once; at either a positive 200 kHz offset or a negative 200 kHz offset, but not both at once. The drawing of dominant first-adjacent interference with a positive frequency offset in FIG. 2 is arbitrary. The circumstance where the dominant interference has a negative frequency offset causes similar interference amounts, except affecting the other sideband.
The permissible analog FM-band interference D/U ratio for first-adjacent interference is only +6 dB. The IBOC DAB signal is typically lower in power than the corresponding analog FM-band signal by between 12 and 30 decibels. In the receiver, the desired and undesired signals are received simultaneously. Thus, first-adjacent analog FM-band signal 17 with first-adjacent center-frequency 15 may significantly occlude upper sideband 11, which is where the desired IBOC DAB signal is presumed to exist. The degree of overlap between analog FM-band signal 17 and IBOC DAB upper sideband 11 is far more significant than the possible overlap between analog FM-band signal 17 and analog FM-band signal 7. The analog FM-band D/U ratio is shown as the difference 19 in received signal unmodulated powers. The D/U ratio in FIG. 2 does not correspond to the worst-case +6 dB D/U ratio. Ratio 19 is meant to be a representation of a first-adjacent circumstance which causes significant IBOC DAB interference and may actually correspond to a D/U ratio substantially greater than +6 dB, and so does not reflect the worst case scenario.
An analog FM-band receiver may operate adequately even when the D/U ratio is as low as +6 dB when the receiver implementation has a sufficiently narrow bandpass filter, as described previously. However, upper IBOC DAB signal sideband 11 is substantially disturbed by the interfering analog FM-band signal 17, and an IBOC DAB receiver which requires that sideband 11 be demodulated and the encoded data determined with a relatively low error rate may not operate properly because of the low effective signal-to-noise ratio (where the noise is the interference signal). As a result, the effective coverage of the IBOC DAB signal may be significantly reduced due to first adjacent interference when compared to the coverage of the corresponding analog-band FM signal, which is undesirable. A prior art IBOC DAB system which requires demodulation of signals which are located in both the upper and lower sidebands around the analog FM-band signal is described in U.S. Pat. No. 5,465,396 to Hunsinger, et. al. Certain other IBOC DAB systems, discussed subsequently, which have been tested by the Electronic Industries Association, also demodulate both sidebands together or demodulate upper and lower sidebands with unrelated source bit information.
Although not shown in prior art FIG. 2, the first-adjacent transmitter may also generate a IBOC DAB signal in addition to the analog FM-band signal. However, the first-adjacent IBOC DAB signal does not significantly affect the desired IBOC DAB signal. Only one of the two IBOC DAB signal sidebands corresponding to the first-adjacent interference source may cause interference (the lower sideband of the interfering transmitter in FIG. 2) because the other sideband is in a substantially different region of frequencies; in other words, significant amounts of interference may only be generated by the interfering IBOC DAB signal sideband (either upper or lower, not both) whichever is closest in frequency to the desired signal. Furthermore, the IBOC DAB signal sideband for the first-adjacent interference source does not significantly overlap the desired IBOC DAB signal sideband in frequency.
For prior art FIG. 2, the lower first-adjacent IBOC DAB sideband (not shown) is situated at a frequency offset between 100 kHz and 200 kHz less than first-adjacent center frequency 15. With respect to center frequency 1, this correspond to an effective offset of between 0 kHz and 100 kHz, which is the region where analog FM-band signal 7 is located and not the IBOC DAB signal. Thus, while the first-adjacent analog FM-band signal is a source of interference to the desired IBOC DAB signal, the first-adjacent IBOC DAB signal is only a source of interference to the desired analog FM-band signal. The interference caused by the first-adjacent IBOC DAB signal to the desired analog FM-band signal is typically not a substantial problem because the IBOC DAB signal power is substantially less than the analog FM-band signal power. For example, when the IBOC DAB power for the first-adjacent interference source is twelve (12) decibels lower than the corresponding analog FM-band power, and since only one of the IBOC DAB sidebands overlaps the desired FM-band signal (which corresponds to a 3 dB reduction in interference power relative to the total IBOC DAB power), the interfering IBOC DAB power will be at least 21 decibels below the desired analog FM-band signal power when the analog FM-band first-adjacent D/U ratio is +6 dB at the edge of the desired signal's coverage. Thus, the analog FM-band interference caused by the first-adjacent IBOC DAB signal is similar to the co-channel circumstance permitted between analog FM-band signals.
Unfortunately, the first-adjacent circumstance causes substantially more interference in the reception of the desired IBOC DAB signal than the desired analog FM-band signal. It is difficult to design an IBOC DAB system which overcomes the high-level of interference that may be created by the worst-case first-adjacent situation. The overwhelming amount of interference is caused by the first-adjacent analog FM-band signal, and not the first-adjacent IBOC DAB signal.
It is a goal of IBOC DAB to supplant the existing network of analog FM-band transmitters over a period of time. As analog FM-band transmitters are turned off or reduced in power, the problems created by first-adjacent interference will diminish. However, during the initial deployment of IBOC DAB, first-adjacent interference will be a serious problem because of the large number of pre-existing analog FM-band transmitters.
Prior art FIG. 3 shows the circumstance where there is second-adjacent interference. It is more likely that there are two significant sources of second-adjacent interference than two significant sources of first-adjacent interference at once because of the greater frequency separation for second-adjacent interference, which allows for a reduction in the physical distance between transmitters. For example, the minimum distance between second-adjacent transmitters is only 40 miles for class B analog FM-band stations, which is substantially less than the required 150 mile separation for first-adjacent class B stations. FIG. 3 is a representation of the interference condition which may be present when there are second-adjacent transmitters located at both a positive 400 kHz frequency offset and a negative 400 kHz frequency offset from center-frequency 1; in other words, interfering analog FM-band signals with center frequency 21, which is f.sub.c +400 kHz, and center frequency 23, which is f.sub.c -400 kHz. The D/U ratios are shown as ratio 25 for the upper (positive offset) second-adjacent interference and ratio 27 for the lower (negative offset) second-adjacent interference. In the second-adjacent circumstance, there is no significant overlap in frequency between analog FM-band signal 7 and upper second-adjacent analog FM-band signal 29 or between analog FM-band signal 7 and lower second-adjacent analog FM-band signal 31. The permissible D/U ratio for second-adjacent interference, with respect to the desired signal with center-frequency 1, is -20 dB so that analog FM-band signals 29 and 31 may have substantially more power than analog FM-band signal 7, determined at the receiver. The larger field strength of the second-adjacent signal(s) when compared to the desired station signal is typically not a significant problem for analog FM-band reception because of the bandpass filtering implemented in the FM-band receiver except for intermodulation distortion, which was described previously.
Although the second-adjacent circumstance does not significantly impact the analog FM-band signal, it is a vital consideration in the design of an IBOC DAB signal. In particular, the potential for second-adjacent interference, as shown in prior art FIG. 3, determines the prac tical outer edges for IBOC DAB signal sidebands 11 and 13. When IBOC DAB is fully deployed, each analog FM-band transmitter will most likely have a corresponding IBOC DAB transmitter, which generates the IBOC DAB signal. When the IBOC DAB signal occupies the upper and lower sidebands around the analog FM-band signal, only one of the sidebands for each of the two possible interference sources may generate significant amounts of interference for second-adjacent allocations. In particular, the IBOC DAB signal in lower sideband 33 of the upper second-adjacent interference with center frequency 21 and the IBOC DAB signal in upper sideband 35 of the lower second-adjacent interference with center frequency 23 may cause interference to the desired IBOC DAB signal in sidebands 11 and 13 (i.e. sideband 35 may interfere with sideband 13, and sideband 33 may interfere with sideband 11).
It is presumed that the shape of the power spectrum envelope of the IBOC DAB signal is about the same for all IBOC DAB transmitters, except for differences in center frequency and transient differences due to specific data patterns. The interfering IBOC DAB signals in sidebands 33 and 35 may be substantially larger than the desired IBOC DAB signal in sidebands 11 and 13 because of the low-valued D/U ratio of -20 dB. In order to prevent excessive amounts of interference, the outer edge of the IBOC DAB signal sidebands 11 and 13 should not exceed a frequency offset of about 200 kHz away from center frequency 1. Then, the outer edge of interfering sideband 35 is approximately adjacent to the outer edge of desired lower sideband 13. Similarly, the outer edge of interfering sideband 33 is approximately adjacent to the outer edge of desired upper sideband 11. In this circumstance, although sidebands 11 and 13 may be surrounded by substantial sources of interference (sideband 33 and sideband 35, respectively), the interfering signals are still approximately frequency-orthogonal and may be separated from the desired sideband signals by bandpass filtering.
The drawing of sidebands 11, 13, 33, and 35 as having a rectangular frequency spectrum in FIG. 1-3 is idealized. It is not practical to implement signals whose spectrum envelope is perfectly rectangular. In general, there may be a small amount of overlap in frequencies between sidebands 13 and 35 and between sidebands 11 and 33 without causing significant degradation of the desired IBOC DAB signal receiver performance (which demodulates sidebands 11 and 13). The amount of frequency overlap that may exist without causing an excessive bit error rate, determined at the receiver, depends upon the method of modulation which is used to generate the IBOC DAB signal in the sidebands. In general, the method of modulation should be optimized together with the shape of the IBOC DAB signal so that the received IBOC DAB signal in the worst-case expected second-adjacent interference circumstance does not cause failure of the IBOC DAB signal receiver. When narrowband modulation methods are used to generate the IBOC DAB signal in the sidebands, for example, "Coded Orthogonal Frequency Division Multiplexing" (COFDM, a.k.a. OFDM, DMT) reference: W. Y. Zou and Y. Wu, "COFDM: an overview," IEEE Transactions on Broadcasting, Vol. 41, No. 1, pp. 1-8, March 1995!, then the outer edge of the IBOC DAB sidebands is less than about 200 kHz because the outer narrowband subcarrier signals may be substantially occluded even when the amount of frequency overlap is small. However, when wideband modulation methods are used to the implement the IBOC DAB signal, for example, "spread-spectrum" modulation reference: R. L. Pickholtz, D. L. Schilling, and L. B. Milstein, "Theory of spread-spectrum communications--a tutorial," IEEE Transactions on Communications, Vol. 30, No. 5, pp. 855-884, May 1982!, then there may be a greater amount of frequency-overlap. Because of the small D/U ratios that are possible for second-adjacent interference, it is important that the "shape-factor" of the IBOC DAB signal be relatively small (i.e. closer to 1 than 2), so that the increase in attenuation at the edges of the IBOC DAB signal sidebands is relatively steep. The IBOC DAB signals which may exist in second-adjacent interference sidebands 33 and 35 typically do not significantly degrade the performance of the receiver for analog FM-band signal 7 because the interfering sidebands are frequency-orthogonal to analog signal 7 and so are substantially attenuated by the IF bandpass filter in the analog FM-band receiver. However, the interfering IBOC DAB sidebands 33 and 35 in the second-adjacent circumstance may cause an increase in the noise level determined in the analog FM-band receiver when the IF bandpass filter is not sufficiently narrow.
During initial deployment of IBOC DAB systems, second-adjacent interference may not be a concern because there will be relatively few analog FM-band transmitters equipped with corresponding transmitters for IBOC DAB signals. This is in contrast to the first-adjacent circumstance, where the large number of pre-existing analog FM-band transmitters may cause significant amounts of interference even upon initial deployment. However, as IBOC DAB systems are more fully deployed to eventually replace analog FM-band systems, second-adjacent interference becomes an important consideration. While attenuating or turning off the analog FM-band signals will substantially diminish the first-adjacent problem in the long-term, the elimination of the analog FM-band signal has no effect on the primary source of second-adjacent interference on the IBOC DAB signal, which is another IBOC DAB signal. When considering the performance of the receiver for the desired IBOC DAB signal, the most severe first-adjacent interference is caused by the first-adjacent analog FM-band signal, and not by the first-adjacent IBOC DAB signal, while the most severe second-adjacent interference is caused by the second-adjacent IBOC DAB signal, and not by the second-adjacent analog FM-band signal.
The deleterious effects of co-channel, first-adjacent, and second-adjacent interference may be present even when there is line-of-sight (LOS) propagation between the transmitter and receiver. However, in many circumstances, particularly when the receiver is mobile, as in a vehicle, multipath propagation may also cause substantial distortion in the received signal. Multipath propagation arises when the LOS propagation path is occluded or when there are strong specular reflectors, which generate echo signals with large amplitudes. As described previously, multipath is generally frequency-selective; the effects vary with frequency. The frequency components which make up the transmitted signal may be altered with varying effects on the amplitude, phase, and delay, which causes distortion in the received signal. For example, multipath propagation may cause the occurrence of narrowband "notches", which are approximate transmission zeroes in the RF channel frequency response. Frequency components of the analog FM-band signal and/or IBOC DAB signal near the notch frequency may be substantially attenuated. The attenuation depth and frequency width of the notches depend upon the echo delay and amplitude characteristics. When the difference in the times-of-arrival of the various received signals is small compared to the reciprocal of the bandwidth of the transmitted signal, then the effect of the notch in the RF channel frequency response is approximately uniform for all frequency components in the signal. This circumstance is known as a "flat-fade". The field strength (i.e. amplitude) of the received signal in the case of flat-fades is often modeled with a "Rayleigh" statistical distribution. The flat-fade typically occurs when there is no LOS propagation path or a LOS path that is significantly attenuated, for example, when obstructed and when the signal reflections are diffuse. Flat-fading may cause a substantial loss in the received signal amplitude. Fortunately, the probability of the occurrence of a loss in signal amplitude of a given amount decreases as the amount of the loss increases. For example, for a single-path Rayleigh distribution, the probability of a loss in amplitude of at least 20 dB with respect to the RMS signal amplitude is about one percent, while the probability of a loss in signal amplitude of at least 30 dB with respect to the RMS signal amplitude is about one-tenth percent. Since the power of the IBOC DAB signal is already small when compared to the corresponding analog FM-band signal power, the effect of Rayleigh fading on the receiver performance may be catastrophic (i.e. weak-signal failure) even with large amounts of error protection redundancy in the IBOC DAB signal.
A known method to combat the deleterious effects of flat-fading multipath is receiver "diversity" reference: W. C. Y. Lee. Mobile Communications Design Fundamentals. Indianapolis, Ind.: Howard W. Sams & Co., 1986, pp. 113-133!. In a receiver diversity system, multiple receiving antennas are used in the implementation of the receiver. In certain circumstances, the antennas are separated by a physical distance which is about one-half of the RF carrier (center-frequency 1) wavelength. In other circumstances, the antennas may be separated by a distance equivalent to many RF carrier wavelengths. By separating the antennas with a sufficient amount of distance, the signal received at each antenna will be approximately uncorrelated and independent of the other received signals. The principle of diversity reception for combating flat-fading is that since the received signals are uncorrelated, the probability of both of the received signals being substantially attenuated by the effects of Rayleigh fading at the same time is substantially less than in the circumstance with only one receiver antenna. A disadvantage of receiver diversity is that it requires at least two receiver antennas.
It is also possible to use diversity at the transmitter site by having multiple (at least two) broadcast antennas. In this circumstance, only a single antenna at the receiver is required. The receiver antenna sums the received signals from the plurality of transmitter antennas. Most analog FM-band transmitters and their corresponding antenna systems do not use transmitter diversity. Furthermore, transmitter diversity systems require that the total licensed power be divided among a plurality of antennas, so that the power available for broadcast in a single antenna is reduced. Diversity may be accomplished with two transmission antennas without physical separation between the antennas by methods known as "polarization" diversity, and "field-component" diversity.
A further known method of diversity is "time-diversity". Time diversity does not require additional transmitter or receiver antennas. In time diversity, two identical messages are transmitted at different instances in time. When the receiver is in motion, as in a vehicle, the received signals at the two different instances in time will be approximately uncorrelated when the time-difference corresponds to about at least one-half RF carrier wavelength. The required distance depends upon the velocity of the receiver so that time-diversity is ineffective at combating flat-fading when the receiver is not in motion. "Interleaving" reference: J. L. Ramsey, "Realization of optimum interleavers," IEEE Transactions on Information Theory, Vol. 16, No. 3, pp. 338-345, May 1970! is a method of time-diversity in which estimates of the determined bits are shuffled by an order-randomizing algorithm in order to substantially mitigate correlated probabilities of error for consecutive bit estimates. Interleaving does not change the probability of error prior to consideration of the error correction code decoding, but it does cause a redistribution of probable errors. Spreading probable burst errors over a larger time interval typically improves the performance of the ECC decoding algorithm, particularly for convolutional codes. A disadvantage to many known diversity systems is that in the corresponding receiver systems, the determination as to which signal path corresponds to a less-affected signal is made by determining a signal-to-noise (SNR) estimate of the signal itself. However, for digital communication systems, it may be difficult to determine the signal SNR estimate with sufficient accuracy. A very small difference in SNR values may result in a large difference in decoded bit error probability.
Prior art systems for new-band DAB and IBOC DAB are known. The Eureka-147 system reference: G. Plenge, ibid.! is a new-band DAB system which makes use of COFDM modulation with a plurality of narrowband carriers. The Eureka-147 DAB signal represents a multiplex of digital audio sources, which are combined, and transmitted together. The minimum occupied bandwidth of the Eureka-147 DAB signal is 1.536 MHz. The operating frequency range for Eureka-147 is not the same for all countries, although a common allocation at L-band has been proposed. The bandwidth of the Eureka-147 DAB signal is significantly larger than that for conventional analog FM-band broadcast, even when considering the upper and lower sideband regions in the RF emission mask. Thus, the Eureka-147 DAB system is not compatible with the narrowband emission and independent transmission characteristics of IBOC DAB. The Eureka-147 system is unrelated to the instant invention.
U.S. Pat. No. 5,465,396 to Hunsinger, et. al., entitled "In-band On-Channel Digital Broadcasting" describes an IBOC DAB method and system where the IBOC DAB signal is generated in the transmitter so that its instantaneous carrier frequency is phase-quadrature to the analog FM-band signal (i.e. AM-over-FM). The IBOC DAB signal bandwidth substantially overlaps that of the occupied bandwidth of the analog FM-band signal. The IBOC DAB signal itself is composed of a plurality of narrowband sinusoid-like signals, which are summed together with a reference signal and combined with the analog FM signal. Since there is considerable overlap in the frequency regions of the IBOC DAB signal and the analog FM-band signal, the signals can be reliably separated only when the IBOC DAB signal and the analog FM-band are phase-orthogonal (i.e. quadrature). It has been previously discussed that the phase-orthogonality may be significantly disrupted by the effects of multipath and frequency-selective distortion. A further disadvantage of the '396 system is that it requires the transmission of an analog FM-band signal (at least the carrier) for proper operation of the IBOC DAB receiver because the carrier frequency estimate and the baud frequency estimate of the IBOC DAB signal determined in the receiver are derived from the analog FM-band signal. Narrowband notch interference which substantially attenuates the carrier and/or pilot frequency component of the analog FM-band signal may then deleteriously affect the IBOC DAB signal, even if most of the bandwidth of the IBOC DAB signal was not affected by the frequency-selective distortion. Finally, the bandwidth of the '396 system is about 400 kHz when there is no analog FM modulation. With modulation, the bandwidth exceeds 400 kHz so that there may be significant second-adjacent interference between IBOC DAB signals.
U.S. Pat. No. 5,278,826 to Murphy, et. al., describes an IBOC DAB system which makes use of a plurality of narrowband COFDM subcarrier signals which are data-modulated with QPSK (i.e. four phase states) modulation and which are simultaneously transmitted. The composite signal is phase-locked to the analog FM-band signal 19 kHz stereo pilot component. The bandwidth of the DAB signal is about 200 kHz and the spectrum of the DAB signal and the analog FM-signal overlap significantly except when there is very little analog FM modulation, which is an uncommon occurrence for conventional analog FM-band signals which are highly processed. The disadvantages of spectrum overlap between the IBOC DAB signal and the analog FM-band signals have been described. In addition to being susceptible to interference from the analog FM-band signal, the system described in the '826 patent is further significantly degraded by first-adjacent analog FM-band interference, which may occupy a bandwidth which substantially overlaps the DAB signal. The '826 patent describes an IBOC DAB system whose subcarrier signal characteristics are similar to the single-sideband COFDM "AT&T Amati LSB" system, described below, but with a larger DAB composite signal bandwidth so that the interchannel interference conditions encountered are more severe. A disadvantage of the '826 system is that when the analog FM-band signal is substantially distorted, the DAB receiver system may not operate properly because it requires the recovery of the analog FM-band signal for IBOC DAB synchronization.
Several new-band and IBOC DAB systems were submitted for laboratory and field testing in the United States. The tests are being conducted by the Electronic Industries Association (EIA) Digital Audio Radio Subcommittee in order to determine a recommendation for a digital audio radio standard. The systems are described in a laboratory test report published by the Electronic Industries Association Consumer Electronics Group (EIA CEG) on Aug. 11, 1995 reference: "Digital Audio Radio; Laboratory Tests; Transmission Quality Failure Characterization and Analog Compatibility," published by Electronic Industries Association Consumer Electronics Group, Digital Audio Radio Subcommittee, Aug. 11, 1995, sections A, AD, AE, AG, AH, AK, AL!. The objective of the EIA CEG was to compare and contrast the performance of the IBOC DAB and new-band DAB systems under a wide variety of circumstances, in order to determine the effects of noise, multipath, co-channel, first-adjacent allocation, and second-adjacent allocation interference on the receiver performance. According to the report, two different IBOC DAB systems were submitted for testing by USA Digital Radio (USADR). These systems are referenced in the EIA report by the system labels "USADR FM-1" and "USADR FM-2". Two different IBOC DAB systems were also submitted by AT&T Corporation in cooperation with Amati Communications (AT&T/Amati) known as "AT&T Amati LSB" and "AT&T Amati DSB". The USADR FM-1 and AT&T Amati DSB systems were modified during the testing process, resulting in system variations with the labels "USADR FM-1 Rev B" and "AT&T Amati DSB Rev B". AT&T Corporation also submitted a DAB system described as being "In-band Adjacent-channel" (IBAC) with the system label "AT&T". An IBAC signal occupies 200 kHz bandwidth, is generated to occupy either the positive-offset first-adjacent frequency allocation (+200 kHz) or the negative-offset first-adjacent frequency allocation (-200 kHz), but not both, and operates at a relatively high power level. Thus, an IBAC DAB signal is not spectrally confined to within the upper and/or lower sideband regions and does not approximately comply with the FCC emission mask. IBAC DAB systems are unrelated to the instant invention.
According to the proponent self-descriptions included in the report, the USADR FM-1 system transmits a IBOC DAB signal, which is composed of a simultaneous multiplex of 48 modulated orthogonal signals together with a reference waveform. Each orthogonal signal, which is a subcarrier signal, spans a bandwidth of about 450 kHz, centered on the analog FM-band center frequency. The subcarrier frequency spectrum has about a 220 kHz void in the center. This is approximately equivalent to having the IBOC DAB signal in the upper and lower sidebands, as described previously. The void in the center of the spectrum causes the IBOC DAB signal to be approximately frequency-orthogonal to the analog FM-band signal, which is desirable. The IBOC DAB signal is unrelated to the analog FM-band signal. The total bit rate of the IBOC DAB signal is 384 kbit/sec and the IBOC DAB signal baud interval is about 125 microseconds. A disadvantage of the USADR FM-1 system is that since each subcarrier signal waveform spans both the upper and lower sidebands simultaneously, large amounts of interference in only one of the sidebands may cause the receiver system to fail by causing a negative SNR ratio for all of the subcarrier signals, whereas in a narrowband system, only some of the subcarrier signals would be deleteriously affected. According to the published EIA CEG test data, the onset of the IBOC DAB receiver system failure, which is known as point-of-failure or POF, for the USADR FM-1 system occurs when the D/U ratio at the receiver is about +7 dB for co-channel interference (versus +20 dB by FCC Part 73), about +22 dB for one-sided first-adjacent interference (versus +6 dB), and about -2 dB for one-sided second-adjacent interference (versus -20 dB). The amount of distortion produced in the recovered audio signal at the POF point is significant. The type of distortion at POF may include silence (mutes), honks, bandwidth-restriction, or extended periods of noise. The EIA data also includes characterizations of the receivers for a performance threshold known as the threshold-of-audibility or TOA. The onset of TOA occurs when lessor audio artifacts (e.g. occasional gurgles, chirps, and whistles) become detectable. Thus, the receiver degradation at TOA corresponds to a milder form of receiver failure than at POF. However, the onset of TOA is typically more rapid than POF. The reference to the POF threshold D/U ratios in this disclosure instead of TOA thresholds is an optimistic measure of IBOC DAB receiver performance. In other words, the recovered audio may be objectionable at a point which corresponds to a less serious noise and/or interference condition than the POF threshold. Although the onset of TOA is more rapid than POF, the difference in the corresponding interference ratios is typically small (e.g. less than four decibels). The first-adjacent (+22 dB) and second-adjacent (-2 dB) D/U ratios for the USADR FM-1 system at the POF threshold are significantly larger than the FCC permitted D/U ratios (+6 dB and -20 dB, respectively). As a result, the first-adjacent and second-adjacent interference-limited coverage of the USADR FM-1 IBOC DAB system may be substantially smaller than the interference-limited coverage of the corresponding analog FM-band signal, which is undesirable.
According to the proponent self-description of the USADR FM-2 IBOC DAB system, the FM-2 system uses phase-orthogonal modulation so that the analog FM-band signal and the IBOC DAB signal are generated in the transmitter to be quadrature. This is similar to the '396 patent to Hunsinger, where the IBOC DAB signal is quadrature to the instantaneous analog FM-band carrier frequency. However, the subcarrier signals in the '396 patent are narrowband while the subcarrier signals used in the FM-2 system are wideband. The FM-2 subcarrier signals are described as being synchronous orthogonal Code-Division Multiple Access (CDMA) signals. Orthogonal CDMA signals are known; see, for example, U.S. Pat. No. 4,460,992 to Gutleber. The bandwidth of the USADR FM-2 IBOC DAB signal substantially overlaps the analog FM-band signal so that separation of the signals depends upon the phase-orthogonality property. As a result, the FM-2 system has significant performance disadvantages when there is multipath or frequency-selective distortion, as described previously. The bandwidth of the USADR FM-2 signal is about 250 kHz, but there are sidebands which extend further out as a result of the "sinc" spectrum shape of the CDMA signals, which is a further disadvantage; in other words, systems with conventional CDMA signals typically have a low spectrum efficiency unless high-order data modulation is also implemented. The total bit rate of the USADR FM-2 system is about 384 kbit/sec and the baud interval is about 500 microseconds. According to EIA data, the D/U ratios at the receiver POF threshold for co-channel, first-adjacent, and second-adjacent interference for the USADR FM-2 system are about +40 dB, about +28 dB, and about +28 dB, respectively. These ratios are substantially larger than the FCC permitted ratios (+20 dB, +6 dB, and -20 dB, respectively), and substantially larger than the corresponding FM-1 system ratios (+7 dB, +22 dB, -2 dB, respectively), so that the coverage of the USADR FM-2 IBOC DAB system may be very poor when compared to the existing analog FM-band signal coverage and the coverage possible with the FM-1 IBOC DAB system. Thus, the FM-2 system appears to be undesirable for DAB.
The remaining IBOC DAB systems described in the EIA CEG report were submitted by AT&T Corporation in cooperation with Amati Communications. These systems are "AT&T Amati LSB" and the revised system "AT&T Amati DSB Rev B". Both systems use Coded Orthogonal Frequency-Division Multiplexing (COFDM), also known as Discrete Multitone (DMT) modulation. In COFDM, a plurality of narrowband orthogonal subcarriers signals are simultaneously transmitted. The simultaneous multiplex allows for the use of a long signaling interval for each individual subcarrier without adversely affecting the system bit rate throughput. Because the bandwidth of each subcarrier is very narrow (typically between about 1 kHz and about 15 kHz), the effects of multipath fading may be modeled as a flat-fade for each subcarrier even when the multipath has specular characteristics. In COFDM, the long signaling interval includes an amount of time known as the "guard interval". When the RF channel multipath delay spread is within the guard interval, then the deleterious effects of intersymbol interference are substantially mitigated.
In the AT&T/Amati IBOC DAB systems, the COFDM signal is generated in the upper and/or lower sideband regions around the analog FM-band signal, as in FIG. 1. The total occupied bandwidth, including the analog FM-band signal, is about 400 kHz. The primary difference between the two AT&T/Amati system configurations is that in the AT&T/Amati LSB system, only one of the sideband signals is transmitted (i.e. either the upper or lower sideband), while in the AT&T Amati DSB system, COFDM signals in both the upper and lower sidebands, with different encoded source bit information, are transmitted. In the AT&T/Amati DSB, the upper and lower sideband signals are unrelated; in other words, the sidebands convey different encoded source bit information. In both configurations, the baud symbol rate is about 4 kHz. According to the proponent self-description, the total bit rate is 192 kbit/sec, and there are about 18 COFDM subcarrier signals in the AT&T/Amati single-sideband (LSB) system. The total bit rate is 256 kbit/sec, and there are 32 COFDM subcarrier signals in the AT&T/Amati double-sideband (DSB) system. The co-channel, first-adjacent, and second-adjacent D/U ratios for IBOC DAB receiver POF threshold are about +15 dB, about +38 dB, and about -2 dB (worst case), respectively, for the AT&T Amati LSB system. Both the first-adjacent and second-adjacent D/U ratios are significantly worse (greater) than the FCC permitted interference ratios of +6 dB and -20 dB, respectively. However, the AT&T Amati LSB system has the advantage that it is insensitive to large amounts of first-adjacent interference when the frequency offset of the first-adjacent source has a polarity opposite to the frequency offset of the one sideband (either upper or lower) which is transmitted. For example, when the first-adjacent interference source is at +200 kHz (15 in FIG. 2), and when the IBOC DAB signal for the AT&T Amati LSB system is substantially confined to lower sideband 13, then the IBOC DAB signal may be robust against the effects of first-adjacent interference, even at high interference levels, because the IBOC DAB signal and the first-adjacent analog FM-band interference source are frequency-orthogonal. Unfortunately, in general, it is not possible to know at the transmitter system (which is where the single-sideband IBOC DAB signal is generated) whether the dominant source of first-adjacent interference at the receiver is due to an analog FM-band transmitter with a positive 200 kHz frequency offset or a negative 200 kHz frequency offset relative to the desired station's frequency because that determination depends upon the physical location of the IBOC DAB receiver (antenna). In a simplex system, such as analog FM-band and IBOC DAB, the transmitter is not aware of the receiver location. Furthermore, in such broadcast communication systems, there are typically a large plurality of receivers at different locations. The different receivers are subject to varying RF propagation and interference effects, so that the resulting D/U ratios, which are determined at the receiver, vary considerably. Another disadvantage of this method is that other sources of interference which may be generated in the same frequency spectrum region as the single-sideband IBOC DAB signal, or which may affect said frequency spectrum region (e.g. multipath), could significantly disturb the IBOC DAB receiver.
The AT&T/Amati DSB system (known as "AT&T Amati DSB") transmits different encoded source bit information in both the upper and lower sidebands. The performance of the revision of the AT&T/Amati system (known as "AT&T Amati DSB Rev. B") is significantly better than the originally submitted system, so that only the performance of the revised system is considered in this disclosure. The co-channel, first-adjacent, and second-adjacent D/U ratios for the IBOC DAB receiver POF for the revised AT&T Amati DSB Rev. B. system are about +9 dB, about +20 dB, and about -20 dB. When compared to the FCC D/U interference levels, the co-channel D/U ratio for the AT&T/Amati DSB Rev B. system at POF is more than adequate (+9 dB versus +20 dB), and the second-adjacent ratio is just adequate (-20 dB versus -20 dB), without consideration of other effects, for example, multipath, which may cause further attenuation of the desired signal. However, the first-adjacent D/U ratio of +20 dB at the receiver POF still differs substantially from the +6 dB FCC-determined ratio.
U.S. Pat. No. 5,359,625 to Vander Mey, et. al., describes a spread spectrum modulation system in which two sidebands are generated by modulating an RF carrier frequency signal with a baseband signal; the frequency spectrum of the baseband signal, which is a spread-spectrum signal, has a passband or bandpass frequency spectrum. In general, the amplitude modulation of an RF carrier signal by a non-zero frequency results in a RF signal with two identical frequency sideband signals with or without an unmodulated RF carrier frequency signal; known as double-sideband large carrier (DS-LC) or double-sideband suppressed carrier (DS-SC), respectively.
A disadvantage of amplitude-modulation systems for IBOC DAB is that the demodulation method for amplitude-modulation, which involves multiplying (mixing) the received signal by the RF carrier signal, inherently combines both sideband signals in determining the baseband signal. Combining the sideband signals in the receiver when one of the sidebands is substantially disturbed may result in a much lower signal-to-noise ratio (SNR) than if one (disturbed) sideband signal is discarded. The use of amplitude modulation alone precludes the representation of different encoded source bit information in the upper and lower sideband signals at the same instant in time.
U.S. Pat. No. 5,369,800 to Takagi, et. al., describes a communication system with identical upper and lower sideband waves (signals). The identical sideband signals are generated by mixing signals with different frequencies in the transmitter system. In the corresponding receiver system, a diversity section determines whether to select between one of the incoming upper or lower sideband waves (signals) or to synthesize a wave from the combination of waves. The '800 system has frequency diversity through the duplication of sideband signals in contradistinction to the duplication of encoded source bit information. Duplication of sideband signals does not necessarily imply duplication of the encoded source bit information, for example, when the bit-to-symbol mapping for the upper and lower sideband signals is not the same. A disadvantage of the '800 system transmitter is that identical upper and lower modulated sideband signals precludes the use of time diversity across the upper and lower sidebands, in which case the sideband signals are not the same at an instant in time. Another disadvantage of the '800 system receiver is that the diversity section in the receiver precedes demodulation and decoding of the source bit information. As described previously, a small difference in SNR values may result in a large difference in decoded bit error rates, and it may be difficult to accurately determine the effective SNR with sufficient precision without use of information obtained after demodulation. Furthermore, the '800 system selects the sideband signal with the greater magnitude. For IBOC DAB, large amounts of interference in a sideband may result in a large apparent sideband signal, the selection of which is disadvantageous.
U.S. Pat. No. 5,265,122 to Rasky, et. al., describes a receiver with antenna diversity. As described previously, multiple receiving antennas (branches) may not be practical for many IBOC DAB receivers. The primary advantage of antenna diversity is the reduction of the deleterious effects of flat-fading and not the reduction of the effects of sideband interference and frequency-selective fading. The '122 receiver system implements a "max-ratio" diversity combiner or "selection" diversity combiner. The combiner weighting parameters are adjusted using coefficients determined from the re-encoding of decoded codewords.
U.S. Pat. No. 5,157,672 to Kondou, et. al., describes an interference detection method for use in an antenna diversity system in a receiver. The disadvantage of antenna diversity for IBOC DAB was described previously. The '672 system determines whether or not there is a substantial amount of interference in a received signal which has been convolutionally encoded by comparing the minimum difference between path metrics, determined by Viterbi decoding, to a predetermined value. A disadvantage of the '672 system is that for convolutional codes with large constraint (K), the minimum difference in path metrics is determined over all 2.sup.k-1 code states, which may be a substantial number of states.
Accordingly, it is apparent from the above that there exists a need in art for a method and system for IBOC DAB which: (i) is able to operate with large amounts of either upper-sideband or lower-sideband first-adjacent interference without a priori knowledge of which sideband is most adversely affected; (ii) is able to operate with large amounts of second-adjacent interference; (iii) is robust against the effects of multipath, particularly short-duration flat-fading; and (iv) provides sufficient bit rate throughput for high quality audio and/or ancillary data services.